Generally, a traveling control circuit of a wheeled hydraulic excavator comprises a variable displacement hydraulic pump driven by a prime mover, a variable displacement hydraulic motor driven by pressure oil discharged from the hydraulic pump, and a control valve for controlling flow of pressure oil fluid supplied from the hydraulic pump to the hydraulic motor. A pair of hydraulic pumps and a pair of control valves are provided. Respective control valves are simultaneously switched to merge pressure oil from each hydraulic pump and supply this merged oil to the hydraulic motor.
The motor rotation speed of this type of wheeled hydraulic excavator is adjusted depending on the amount of depression of the accelerator pedal. In addition, it is possible to change a gear ratio between the output shaft of the hydraulic motor and a vehicle wheel using a transmission. For example, the transmission sets a low speed range or a high speed range to obtain a gear ratio in two levels. In this way, it is possible that the vehicle travels at a wide range of speeds from low speed travel for a work site to statutory speed travel (high speed) for public roads.
With the above described wheeled hydraulic excavator, an ultra low travel speed (for example 2 km/h) may be required which is similar to the speed of a hydraulic excavator mounted with a crawler. That is required for a mowing operation, for example, using a mowing unit which is fitted as a working attachment for the wheeled hydraulic excavator. The ultra low travel speed is achieved by switching the transmission to a low-speed range and adjusting the amount of depression of the accelerator pedal, however, it is difficult and imposes a significant burden on an operator.